In general, when cement clinker is calcined by a SP (suspension preheater) kiln or a NSP (new suspension preheater) kiln, volatile components such as chlorine, alkali, sulfur etc. brought from cement raw material and fuel are sequentially concentrated by circulating in the kiln preheater.
However, it is known that the circulation reaches equilibrium in a few hours so that the amount of the volatile components brought into a system from the cement raw material and fuel becomes equal to the amount of the volatile components taken out to the outside of the system by the cement clinker.
When a large amount of the volatile components is brought by the raw material and fuel in this case, the amount of the volatile components in the clinker is increased, by which the quality of cement is adversely affected.
Further, when the amount of the volatile components is increased in the system, since compounds of low melting point are made, the preheater is often clogged, by which the stable operation of the kiln is obstructed.
Recently, when the effective utilization of industrial waste is promoted in particular, a raw material containing a large amount of chlorine cannot help being used, thus it is desired to effectively remove the volatile components.
To cope with this problem, there is employed a so-called alkali bypass system to reduce the amount of the volatile components in the kiln preheater system. This is a method of extracting kiln exhaust gases containing highly concentrated volatile components to the outside of the system by an alkali bypass system. Next, the alkali bypass system will be briefly described.
Kiln exhaust gases of about 1100.degree. C. extracted from a kiln through an extraction duct are introduced into a cooling chamber and mixed therein with cooling air from a fan so as to reduce the temperature of the gases to 400 to 450.degree. C. At the time, the compound of the volatile components is condensed on the surface of dust by the fall of the gas temperature.
Further, after the temperature of the gases is lowered to about 150.degree. C. by spraying water to them in a conditioning tower, the dust in the gases is collected by an electrostatic precipitator and remaining gases are exhausted into the atmosphere through a fan.
Although the dust is collected by the conditioning tower and the electrostatic precipitator, since the dust contains the volatile components concentrated thereto, it is disposed of as waste.
However, in the above method, since the kiln exhaust gases of about 1100.degree. C. are exhausted to the outside of the system, heat loss is greatly increased.
Further, because a large amount of the dust is discharged to the outside of the system and disposed of as waste, the disposal of it becomes difficult year by year due to the shortage of places where the dust can be disposed of and the change of the awareness of inhabitants who live in the vicinity of the places where the dust is disposed of.
In order to solve such problems, Japanese Patent No. 1835995 and Japanese Patent No. 1702995 disclose methods of reducing heat loss and the amount of dust to be disposed of.
These patents disclose a method of setting a gas cooling temperature to 600.degree. C. to 700.degree. C., collecting the dust in gases by a dust collector and returning exhaust gases to a preheater exhaust gas system to thereby collect the heat thereof by as waste heat boiler so as to reduce the heat loss of the exhaust gases, and a method of cooling extracted gases to 600.degree. C. to 700.degree. C., collecting the heat of the gases by a dedicated boiler, and then collecting the dust in the gases by a dust collector and exhausting the exhaust gases to the outside of a system. In addition, these patents realize reducing the absolute amount of the dust to be processed by causing the extracted gases whose temperature is made to 600.degree. C. to 700.degree. C. to pass through a separator so as to return the dust having a size of 10 .mu.m or more to a kiln as it is.
In short, a gist of the above patents is to reduce heat loss and lower the cost of an apparatus by finding that the temperature of extracted gases can be increased from conventional 400.degree. C. to 450.degree. C. to 600.degree. C. to 700.degree. C.
Another gist of the patents is to reduce the amount of the dust to be disposed of by separating the dust whose size is greater than 10 .mu.m by the separator and returning it to the kiln by finding that alkali is omnipresent to fine particle size.
That is, this invention is embodied by correctly grasping the condensing temperature of volatile components, discovering that a larger amount of alkali is distributed to the fine particle size of dust, and realizing the discovery.
The prior art intends to mainly remove alkali and a large amount of kiln exhaust gases must be extracted to achieve the object. Even if the two patents described above are embodied, the heat loss of a kiln/preheater system will be about 140 to 180 J/kg-clinker in a suspension preheater kiln and about 50 to 70 J/kg-clinker in a new suspension preheater kiln when 10% of kiln exhaust gases is extracted. The largest factor causing the heat loss is that the kiln exhaust gases are extracted in a large amount.
Further, since the amount of discharged dust is increased in proportion to the amount of the extracted gases, to employ a suitable method of processing the dust is also a large problem.
Taking the above circumstances into consideration, an object of the present invention is to economically and stably operate a kiln with a less amount of heat loss. Another object of the present invention is to simply dispose of fine particle dust.